Fluid boiling and condensing heat transfer system



Aug' 8, 1957 F. BURGGRAF ETAI. 3,334,685

FLUID EOILING AND CONDENSNG HEAT TRANSFER SYSTEN Filed Aug. 18, 1965 United States Patent O 3,334,685 FLUID BOILING AND CONDENSING HEAT TRANSFER SYSTEM Frederick Burggraf, Cincinnati, Ohio, and Archie Harold Perugi, Malvern, Pa., assignors to General Electric Company, a corporation of New York Filed Aug. 18, 1965, Ser. No. 480,645 4 Claims. (Cl. 165-105) The present invention is directed to a uid boiling and condensing heat transfer system and, more particularly, to such a system for use on a structure to be cooled, which system conveniently uses sealed tubular liquid metal cooling means better known as a thermosyphon.

In many applications the limiting factor to the usev of extremely high temperatures is generally the material that is available. In order to use available materials it has become necessary to devise various cooling systems to cool the structure which is to be operated at the lhigh temperatures. A typical cooling -arrangement that has been found to have useful applications is the thermosyphon arrangement which is a closed or sealed tube in which a liquid or gas may be disposed. r[The tube may be disposed in a centrifugal field in those cases where it is associated with rotating parts. Such an all-liquid system or all-gas system may use a single fluid and may cool by convection circulation wherein uid is heated in one end of the tube and, being warmed, rises to the other end of the tube where it is cooled and falls back to the original tube end. This sets up a convection circulation and results in a -iiow liuid from one end of thetube lto the other. This is what is known as free convection cooling and such cooling is well-known. Another type of cooling system is a boiling and condensing system Where, using the same structure, liquid may be boiled in the heated or boiler part of the tube and then the vapor rises and flows to t-he cold part of the tube at the other end where it is condensed on the walls in the condenser portion. The liquid then falls back to the hot boiler end of the tube where it is again re-vaporized and the cycle continues. 'I'he division between t-he boiler and condenser is termed the interface. Such boiling and condensing systems may use liquid metal and are well-known.

One of the diiculties of the latter system is that it does not always operate at its maximum theoretical heat transfer capacity. A parameter which limits total heat transfer performance is the net vapor ow area available for ow from the boiler at the lower end of the tube to the condenser at the other end of the tube. Additionally, the incorrect amount of theoretically proper fluid results .in poor heat transfer in that the extra fluid in the liquid form may tend lto settle in the lower end of the tube when too much' uid is supplied. On the other hand, if too little fluid is supplied then the liquid will be completely vaporized before the returning condensed vapor refills the liquid supply with the result that the lower end l of the tube may become dry, overheat, and melt. In other words, 'the correct amount of iiuid results in a continuous cycle wherein the returning condensed vapor is immediately boiled to recirculate so that there is no built-up liquid supply or puddle and the system is completely balanced for maximum heat transfer capacity. In order to obtain this desirable end, it is desirable to obtain nucleate boiling off the surface of the tube. Nucleare boiling may be described as a form of boiling where the liquid is maintained on the surface and the gas bubbles that form come through the surface but do not break the liquid free from the surface. Providing this phenomenon results in very high heat losses or heat transfer such that it is possible to take a large amount of heat out of the surface of the tube. The key to maintenance of nucleate boiling is to keep the liquid iilm relatively thin so that the 3,334,685 Patented Aug. 8, 1967 gas bubbles that form or evolve come through the liquid surface and break free into the Vapor atmosphere and do not take the liquid film with them. As opposed to this, 4-lilm boiling permits the formation of gas onthe surface, and when it leaves the surface, it lifts the liquid away from the surface to leave a igas layer thereon which insulates and reduces heat transfer capacity. Further boiling cannot take place until the liquid is returned to the surface. It is desired to maintain nucleate boiling for the highest heat transfer capabilities so that a thin liquid film owing down the wall vaporizes off the wall and back into the vapor or gas form to immediately iiow to the cold end of the tube and condense on the wall. The amount of fluid supplied must be just suicient to fill the system and provide this thin liquid film down the wall without any excess to reduce 4the heat transfer and without a shortage to result in burning of the system.

The main object of the invention is to provide a fluid boiling and condensing system that is substantially completely balanced with the right amount of liquid to maintain Athe proper thin film on the tube surface.

Another object is to provide such a system wherein the nucleate boiling is ensured along with condensing for maximum heat transfer capabilities.

A further object is to provide such a system wherein the uid supplied is accurately predetermined in order to provide the amounts necessary for a balanced system and thus avoid lower heat transfer capacity in the event gf too much fluid and burn-out in the event of too little uid.

A further object is to provide such a system that may be applied to a rotating structure, such as a turbine bucket, and wherein the system is so balanced that it may use a canted tube with the proper amount of fluid therein.

Another object is to provide such a system which may employ differing size tubes such as may be encompassed by tapered tubes and wherein the velocity of the fluid at the interface between the vapor and the liquid does not exceed sonic velocity.

Briey stated, the invention comprises a uid boiling and vcondensing heat transfer system that may be used on a structure to be cooled. The system comprises a sealed tubular member within the structure, the member containing a iiuid such as a liquid metal although the system is not limited to liquid metal. The tube is oriented to provide a boiler at one end and a condenser at the other end and is sized to have a maximum of sonic velocity at the interface between the boiler and the condenser in accordance with the heat to be transferred at a `given temperature from the member to the liquid for vaporizing the liquid. For a certain temperature level at the interface and a certain maximum heat flow, the sizing ofthe tube is `determined in accordance with a formula that provides choked ow at the interface. Further, the minimum amount of liquid provided in the system is made up of the summation of the weight of uid in its vapor state and the weight of fluid in its liquid state as determined by specific formulae.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed the invention will be better understood from the following description taken in connection with the accompanying drawings, in which:

FIGURE 1 is a diagrammatic cross-sectional view of a typical boiling and condensing heat transfer tube;

FIGURE 2 is a partial perspective view of the system as it may be applied to a turbine bucket;

FIGURE 3 is a diagrammatic showing of an area changing tube being shown in the normal tapered form; and

3 FIGURE 4 is a modification of an area changing tube over that of FIGURE 3 showing another form within the -term tapered Referring rst to FIGURE 1, there is shown a general form of known boiling and condensing system with g forces shown by the block arrow, which may comprise a tube of any cross-sectional shape supplied with a uid 12 and which, for convenience, is described herein as a liquid metal theormosyphon as it might be applied to a turbine bucket 13 as shown in FIGURE 2 but may be applicable to any structure that is designed to be cooled at one portion and has means for taking heat away at another portion. The turbine bucket is a convenient execution and a normal application for the system. The liquid metals may preferably be used because of their high heat transfer capabilities. If the temperatures are proper, any suitable uid may be employed.

In the normal operation of the boiling and condensing system, the iiuid 12 is boiled by the addition of heat as shown by the arrows 14 and this may be due to the hot gases passing over the turbine bucket. Boiling of the uid 12 creates a vapor which passes as shown by the heat arrows '15 from the boiler 16 to the other end of tube 18 which is a condenser. At this end of the tube, the heat is removed by any suitable means, e.g., a cooling uid, as presented by the arrows 20. The removal of the heat, of course, condenses the fluid on the walls of tube 10 and the fluid ows down the walls to the supply 12 and the cycle is repeated. This is a common and wellknown boiling-condensing liquid metal system.

It has been found desirable that the supply of fluid 12 be kept substantially exactly correct at a given temperature condition to be handled in order that the system may be completely balanced for maximum heat transfer capacity. In other Words, if too much fluid is supplied at 12 then it reduces the heat transfer capacity inasmuch as the returning liquid on the walls of the tube must lie in a pool before sufficient heat is added to vaporize the fluid again. The desired condition is to have the iiuid returned on the tube wall and immediately vaporize that there is never any built-up supply or puddle of uid at 12. On the other hand, it is desired to have sucient fluid so that it is not all boiled away from 12 before the returning uid reaches 12 as it comes down the walls. In that situation the tube would clearly melt or burn.

The instant invention proposes a two-fold improvement on such a known system. Summarized, this consists in providing the correct flow area and the correct amount of liquid to meet these objects.

One of the parameters which limits the total heat transfer performance in such a system is the net vapor flow area available for flow from the boiler at one end of the tube to the condenser at the other end of the t-ube.

In the boiler portion of the tube, heat is transferred to the liquid flowing along the tube walls towards the boiler end. This heat causes the liquid to vaporize and generate a vapor which is forced towards the condenser end of the tube by vaporization of successive quantities of liquid passing along the walls of the boiler section. As the vapor flows from the boiler to the Icondenser portion, its flow rate is increased by additional vapor which is generated along the length of the boiler portion walls. The ow rate of the vapor continues to increase until no additional vapor is added to the vapor ow. This point is generally found at the interface 22 between the boiler and the condenser. The interface 22 may be the physical separation point between the boiler portion and the con- -denser portion of the heat exchanger or the point at which no heat is being transferred to or from the fluid. By increasing the amount of heat transferred to the fluid in the boiler section, the maximum flow rate of the vapor is increased.' However, the flow rate of the vapor is limited to a condition wherein the velocity of the vapor at the interface is sonic or choked.

The maximum heat transferred during this condition is H :PsatA tCL wherein:

H=the heat transferred from the boiler portion in B.t.u.s/hour psat=the density in pounds/cubic foot of the vapor at saturation At=the cross-sectional .area in feet squared of the tube at the interface between the boiler and the condenser portions L=the latent heat of vaporization in B.t.u.s/pound of the uid circulating between the boiler and condenser portions C=the sonic velocity in feet per hour of the vapor at saturation Therefore, with the above formula, it is possible to determine the minimum cross-sectional area of a tube to enable transfer of a -given maximum heat capacity. The above formula enables selection of a minimum crosssectional area for use in meeting the maximum anticipated heat transfer requirements.

Thus, at a given temperature, for example l400 F., if it is desired with a normal straight tube as shown in FIGURE l to coolor prevent the temperature from exceeding l400 F. then it may be calculated how many B.t.u.s must be removed 4and the required area At of the tube may be determined from the above formula.

This minimum area requirement becomes increasingly important when the tube is incorporated in a turbine bucket of a gas turbine engine. In this case, it is important that the cross-sectional area of the cooling tube be at a minimum because of the relatively thin width of the blade. In order to further minimize the area taken up by the tube, the walls of the tube are tapered from the interface to the boiler and condenser portion respectively.

It will be apparent that a tapered tube, such as shown in FIGURE 3, can better accommodate these vapor flows than can a straight lwalled tube. In this sense, the term tapered as used herein is intended to include a varying area sized tube such as shown in FIGURE 3 or the modi- -fied stepped portion as shown in FIGURE 4 or equivalents wherein the area is varied for the purpose herein. Any means that provides a variable area within the tube is intended to fall within this term tapered since it will be apparent that FIGURE 4 represents a series of holes with progressively increasing size to the center of the tube. By this tapering arrangement, boiler vapor ows can be accommodated while maintaining suitable diameters in individual regions.

The result of the orientation of the tube with the boiler at one end land the condenser at the other end as explained in the sizing according to the above formula, is that a tube is pro-vided with a minimum area and a maximum heat transfer capacity at `a given temperature. Additionally, it is important that the velocity at noplace over the tube length exceeds sonic. This may be obtained by sizing the tube and tapering from the interface according to the above formula so the taper produces a maximum of sonic velocity at the interface and the result, in part, provides for nucleate boiling and assists in obtaining maximum heat transfer.

An additional important feature is the amount ofV liquid till and its effect on the tube performance. There isa minimum fill level which will permit operation at maximum heat transfer capacity. With increased heatV input beyond this minimum the tube is subjected to a condition where all of the liquid metal fill enters into the circulating boiling-condensing ow process and a small amount of additional heat 'would result in no liquid available for vaporization and thus absorption of heat.V

Under this condition as previously noted, the end of the boiler will be dry yand temperatures will rise rapidly resulting in tube melting. In order words, there is a correct minimum iill required for the complete balance for maximum heat transfer. Within the tolerance of this amount, which is a minimum, a slight additional amount of iiuid may be added to take care of manufacturing tolerances.

The minimum ill of uid, that is needed to lill the vapor and liquid ow system for operation under the conditions described, is the sum of the weight of fluid in the vapor state plus the weight of fluid in the liquid state. The weight of fluid in the vapor state in pounds in the system can be obtained by summing up the densityvolume products over the entire tube length according to the formula:

Fluid vapor Weight in pounds (g) The above must be made as a summation since there is a calculable temperature variation over the system length so that the vapor density will vary from the condenser base to the boiler tip.

The weight of the Huid in-.the liquid state is determined by the heat ux to be transferred at the particular design tube operating condition which will dictate maximum thickness at the boiler-condenser interface plane. Thickness above and below this plane is dictated by heat ux distribution as received by .the boiler and as dissipated by the condenser. The normal uid ow equations for a falling iilm with friction at the Wall can be used to determine fluid velocity along the surfaces. Thus, the weight of the iiuid in the liquid state in pounds is determined as follows:

Fluid liquid Weight in pounds (g boiler tip p11) 5dr condenser base where;

pL=liquid density, pounds/ft.3 P=tube perimeter in feet =lm thickness' in feet x=incremental length (dx) in feet Both of the -above uid vapor Weight and fluid liquid weight are integrated between the same limits, i.e., the condenser base and the boiler tip.

The sum of these two weights of uid in the vapor state and uid in the l-iquid state will then be the minimum duid required for proper balanced operation of the boiling and condensing system for maximum heat transfer. This is illustrated in FIGURE 5 which shows the uid iill level for a tube with la const-ant diameter. For tubes with tapering diameter, the curves would be expressed in terms of uid till level as a percentage of tube length versus the tube volume.

The maximum fill level (also shown in FIGURE 5) is dictated by the heat ilux distribution near the tip of the boiler. Obviously, some liquid over t-he minimum calculated amount is needed to account for slight variations in manufacturing of the small tubes and in the illing procedures and variations in the application for different operating conditions. The surplus of uid over the minimum amount will be taken by the -rotating g forces to the tip of the boiler when the tube is in a rotating system as a turbine. This excess of liquid will remain there during operation in the rotating system. -If too much liquid is carried in the tip of the boiler Ia large increase in temperature occurs due to the liquid head. The maximum iill level will be made up of the minimum weight of flu-id required for proper system balanced operation as just described plus a weight of fluid which,

6 during operation, will fill the tube to the thickness of the thermal boundary layer of the hot gas lowing over the boiler tip. This thickness may be of the order of 10% of the boiler height for a typical application.

It should be noted that the tube need not be completely radial to the axis of rot-ation 24 as shown in FIG- URE 2 although, in practice, it will generally be so oriented. However, operation may take place with the tube canted as in the turbine bucket up to a maximum limit of 70 from the Iradial to the axis of rotation of the bucket or the horizontal reference axis. In other words, the tube may, if constructed and filled as described above, be tipped to as low las 20 off the horizontal and still operate as desired. v

-In summary, the boiling and condensing heat transfer system described provides a system which sizes the tube correctly so that at no point is sonic velocity exceeded and it provides the maximum area of the tube generally at the interface between the vapor and liquid phases. In addition, the invention provides a minimum fill or supply of liquid so that the system operates balanced Without an excess capacity to reduce the heat transfer capabilities. The system of the invention may be applied to straight or tapered tubes -although the tapered tubes vgenerally provide for better v-apor ow for best heat transfer.

While there has been described a preferred form of the invention, obvious equivalent variations are possible especially in -the structural form of tapering in light of the above teachings. It is therefore .to be understood that within the scope of the appended claims, the invention m-ay be practiced otherwise th-an as speciiically described, and the claims are intended to cover such equivalent variations. v

We claim:

1. A fluid boiling and condensing heat transfer system for use on a structure to be cooled comprising;

a sealed tubular member within the structure, a uid disposed in .the tube, said tube being oriented to provide a boiler at one end for vaporizing said uid and a condenser at the other end and bein-g sized to have a minimum area and sonic velocity of the Vaporized uid at the interface betweenP boiler and condenser portions in accordance With the heat to be transferred at a given temperature from the member to the liquid for condensing said liquid such that:

At=H/pCL where:

At=tube cross-sectional area in ft.2 of the tube at the interface between said boiler and condenser portions,

H :desired heat transferred from the boiler portion in B.t.u.s/hr.,

psat=density of the vapor in pounds/ ft.3 a-t saturation,

C=sonic velocity in feet/hour of the vapor at saturation,

L=latent heat of vaporization of the uid at saturation in B.t.u.s/ pound,

and the minimum 'amount of said duid in the tube is the sum of the uid vapor weight (g) and the ilui-d liquid weight (g') where;

Fluid vapor weight in pounds (g) boiler tip f PvAtdx condenser base where:

pvl=vapor density in pounds/ft.3, At=tube cross-sectional area in it?, x=incremental length (dx) in feet,

integrated between the limits of the condenser base and the boiler tip and;

'Fluid liquid weight in pounds (g')=fpLPdx where:

pL=liquid density in pounds/f, P=tube perimeter in feet, =f1lm thickness in feet, x=incremental length in feet,

integrated between .the same lim-its whereby the desired amount of liquid 'for boiling and condensing is supplied -for maximum heat transfer at the given temperature.

2. Apparatus as described in claim 1 wherein said structure to be cooled is a rotating turbine bucket, having a bladed portion across which relatively hot gases pass,

said tube is positioned so that said boiler portion extends into said bladed portion of said :turbine bucket and, sai-d condenser portion extends inwardly of said bladed portion for passage of cooling iluid thereacross.

3. A liquid metal boiling and condensing heat transfer system for use on a struct-ure to be cooled comprising;

a sealed tubular 'member within the s-tructure, a liquid metal disposed in the tube, said tube being oriented to provide a boiler at one end for vaporizing the :dui-d and .a condenser -at the other end and being sized to have a minimum area and sonic velocity at the interface between said boiler :and condenser portions and said tube being substantially tapered from said interface, thelimits on said taper being that which produces at any point along ithe tube sonic velocity yas a maximum, said minimu-m area sizing being in laccordance with the heat to be transferred at a given temperature from the member to the liquid for boiling said liquid such that; At-TH/psatCL in feet where:

At=crosssectional area in ft2 of .the tube at the interface between the boiler land condenser portions, y

H :desired heat transferred from the boiler portion in B.-t.u.s/hr.

psat=density of the vapor in pounds/ft? at saturation,

:sonic velocity in feet/hour of the vapor at saturation,

L--latent heat of vaporization of the l uid at saturation in B.t.u.s/pound,

and the minimum amount of said liquid in the tube is the sum of the uid vapor weight (g) and the uid liquid weight (g) where;

Fluid vapor Weight in pounds (g) boiler tip pvAtdx condenser base pv=vapor density in pounds/ ft, A5=tube cross-sectional area in ft?,

x=incremental length (dx) in feet,

integrated between the limits of the condenser base- .and the boiler tip and;

Fluid liquid weight in pounds (g fpLPclx where:

pL=liquid density in pounds/ft-3, P=tube perimeter in feet,

=lm thickness in feet, x=incremental length in feet,

across.

References Cited UNITED STATES PATENTS 2,529,915 11/1950 Chausson 165-105 X 2,548,092 4/1951 Bartlett et al. 16S-105 X 2,813,698 11/1957 Lincoln 165 105 X FOREIGN PATENTS 633,242 12/ 1961 Canada. 1,002,570 2/ 1957 Germany.

ROBERT A. OLEARY, Primary Examiner.

MEYER PERLIN, Examiner.

A. W. DAVIS, IR., Assistant Examiner. 

1. A FLUID BOILING AND CONDENSING HEAT TRANSFER SYSTEM FOR USE ON A STRUCTURE TO BE COOLED COMPRISING; A SEALED TUBULAR MEMBER WITHIN THE STRUCTURE, A FLUID DISPOSED IN THE TUBE, SAID TUBE BEING ORIENTED TO PROVIDE A BOILER AT ONE END FOR VAPORIZING SAID FLUID AND A CONDENSER AT THE OTHER END AND BEING SIZED TO HAVE A MINIMUM AREA AND SONIC VELOCITY OF THE VAPORIZED FLUID AT THE INTERFACE BETWEEN BOILER AND CONDENSER PORTIONS IN ACCORDANCE WITH THE HEAT TO BE TRANSFERRED AT A GIVEN TEMPERATURE FROM THE MEMBER TO THE LIQUID FOR CONDENSING SAID LIQUID SUCH THAT: 